Gate last high-k metal gate (HKMG), also called replacement gate, techniques have been developed to address problems attendant upon substituting metal gate electrodes for polysilicon gate electrodes. For example, an amorphous silicon (a-Si) or polysilicon gate is used during initial processing until high temperature annealing to activate source/drain implants has been implemented. Subsequently, the a-Si or polysilicon is removed and replaced with a metal gate.
Replacement gates are being employed, especially in 32 nm, 28 nm, and 22 nm technology nodes. However, “metal gate voids” result from the gate metal fill processes, particularly at such small pitches. Adverting to FIG. 1, after polysilicon has been removed, the gate profile 101 exhibits a bottle-neck shape 103 at the top. During the metal fill process, the bottle-neck part “pinches-off” by the gate metal (titanium nitride (TiN) barrier 105 and aluminum (Al) 107) before the bottom of the gate is completely filled with metal. Also, TiN and aluminum Al tend to fill thicker at the top of the gate than at the bottom of the gate. This causes incomplete metal-gate fill, or voids 109 in the metal gate. Voids cause work function issues, such as variations in Vt, as well as high gate resistance and ac Reff penalty.
Attempts have been made to fabricate substantially vertical gate profiles to eliminate voids. However, a vertical profile is difficult to obtain with a reactive ion etch (RIE) process. Further attempts have been made to fill the barrier metal and Al gate metal substantially conformally to eliminate voids. However, TiN and Al (especially Al) do not readily fill conformally.
A need therefore exists for improved methodology enabling formation of void-free metal gates.